1. Field of the Invention
This invention is directed generally to rubber compositions for automobile tires and has particular reference to such a composition for use in tire treads of an all-weather type.
2. Description of the Prior Art
From the safety viewpoint of automotive vehicles while in running particularly on snowy and icy roads, attempts have been made with a plurality of spikes placed peripherally around a tire tread, or with an antiskid chain held in surrounding relation to a tire body. Both skidproof means have a drawback in that upon biting into snow and ice during running of the tire, they tend to come into frictional contact with the pavement. This causes dust and dirt in a thawing or dry season, eventually posing hazardous environmental pollution.
In Japanese Patent Laid-Open Publication No. 63-172750 a certain rubber composition has been proposed for use in an attempt to eliminate or alleviate these safety and pollution problems. This prior composition has dispersed, in addition to a matrix rubber, a rubber component of low hardness resulting from vulcanization and subsequent comminution so as to improve abrasion resistance and heat buildup and hence friction force on snow and ice.
Numerous other compositions of the character mentioned are disclosed for instance in Japanese Patent Laid-Open Publication No. 53-133248, No. 55-135149, No. 58-199203, No. 59-142236, No. 60-44538 and No. 60-137945.
In general, studless tires are provided in their tread patterns with a multiplicity of blocks in which small grooves commonly called sipes are disposed substantially in parallel relation to the axis of rotation of the tire, thereby forming extra traction portions and preventing snow and ice skids. Although acceptable in runnability on snow and ice, the foregoing composition of Japanese Publication No. 63-172750 when used in the a tread portion of such studless tire is rather small in hardness and hence poor in durability under dry and wet running conditions. To be more specific, cracking is liable to take place and eventually grow at the bottom of many of the recesses between and among the blocks and also at the bottom of the sipes. On exposure of the tire to a braking force as opposed to a driving force, the tread blocks readily collapse due to shear stresses and become weak. Thus the composition taught by the publication in question has been found to have only a limited utility. Similar considerations apply to the compositions appearing in the other publications.